Project Summary/Abstract Planar Cell Polarity (PCP) signaling polarizes cells in epithelial sheets along an axis orthogonal to their apical- basal axis, and is necessary for both developmental and physiological functions. Most of our mechanistic understanding of PCP signaling derives from work using Drosophila as a model system. However, a range of medically important developmental defects and physiological processes in vertebrates are under control of PCP signaling, motivating considerable interest in studying PCP both in Drosophila and in vertebrate model systems. Studies in the fruitfly, Drosophila, from my lab and others, reveal a modular system controlling PCP, in which the functional modules each comprise a genetically and biochemically related unit. Evolution has apparently adapted these mechanisms to perform diverse functions in vertebrates. Genetic analyses have identified many processes controlled by orthologs of fly PCP genes, and that are therefore presumed to function by conserved mechanisms. However, while in some vertebrate tissues, characteristic features of the PCP signaling mechanism identified in flies are evidently conserved, it is also apparent that in various contexts, the mechanism has been adapted and evolved, and therefore that not all developmental events that require PCP genes necessarily function by the same mechanism. Furthermore, while Drosophila has one, or in a few cases, two, paralogs of essential PCP genes, vertebrates typically have three, four or more paralogs, suggesting the possibility of diversification of function within a single polarization process. Here, we propose to continue our studies PCP in the mouse tracheal epithelium. PCP regulates the orientation of motile cilia in many tissues. We have established the mouse tracheal epithelium as a powerful model for this function, complementing intact animal genetics with the ability to grow this epithelium in culture. We will use this system to look closely at signaling mechanisms and their relationship to developmental events. Because vertebrates have multiple paralogs of single fly PCP genes, we will use this system to explore the diversification of gene function, and to study how PCP signaling is translated into polarization of motile cilia.